A large number of liquid or solid acid catalysts are known for use in carrying out the aliphatic alkylation of isoparffin(s) such as isobutane or isopentane using at least one olefin such as propylene, 1- and 2-butenes or isobutene. The catalysts which are most frequently used in the industry are liquid catalysts, namely concentrated sulphuric acid and hydrofluoric acid, used alone or mixed with Lewis acids such as boron trifluoride. These processes suffer considerable disadvantages: hydrofluoric acid because of its toxicity and high volatility; and sulphuric acid because of its high consumption of catalyst, necessitating costly reprocessing. For this reason, the use of solid catalyst or catalyst supported on solids such as aluminosilicates or metallic oxides, for example zirconia treated with sulphuric acid, has been recommended.
In order to catalyse the alkylation of aromatic compounds by at least one olefin or the aliphatic alkylation of isobutane by at least one olefin or the oligomerisation or polymerisation of olefins, U.S. Pat. No. 3,795,712 describes catalytic compositions comprising a Lewis or Bronsted acid and a sulphone with formula R--SO.sub.2 --R', where R and R' are each and separately a monovalent radical comprising 1 to 8 carbon atoms per molecule, or together form a divalent radical containing 3 to 12 carbon atoms per molecule, optionally in an inert hydrocarbon solvent. The acid concentration is in the range 10.sup.-5 moles per litre of sulphone to 5 moles per litre of sulphone, i.e., in the range 0.00012% to 37% by weight of acid when the acid is trifluoromethanesulphonic acid and the sulphone is sulpholane.
International patent application PCT WO 93/00316 describes catalytic compositions for aliphatic alkylation comprising 10% to 90% by weight of hydrofluoric acid or sulphonic acid substituted with at least one halogen and 10% to 90% by weight of a solvent with a donor number of less than 40 in the absence of a deliberate addition of a metal halide. The long list of such solvents includes sulpholane (tetramethylene sulphone) and dimethylsulphoxide.